1. Field of the Invention
The present invention generally relates to image forming methods and image forming apparatuses.
2. Description of the Related Art
An image forming apparatus having the following structure and mechanism is known. That is, toner images, as developer images, each formed on a photosensitive body (image carrier), are sequentially overlapped (superposed) on an endless belt intermediate transfer body at multiple times for primary transferring. Multicolor toner images formed by stacking the toner images on the intermediate transfer body undergo secondary transfer to a transfer member (for example, a transfer paper).
In such an image forming apparatus, timing when the toner images transferred onto a surface of the intermediate transfer body reach a secondary transferring position where the toner images undergo secondary transfer is matched with timing when the transfer member is conveyed to the secondary transferring position, so that secondary transferring can be achieved by matching a head end of the toner image on the surface of the intermediate transfer body with a head end of the transfer member.
The timing when the toner image transferred onto the surface of the intermediate transfer body reaches the secondary transferring position can be calculated based on a set value of the perimeter length of the intermediate transfer body. However, the perimeter length of the intermediate transfer body may be changed based on change of environment such as temperature change or humidity change or degradation with time.
If the perimeter length of the intermediate transfer body is changed so as to shift from the set value, an error may be generated between the timing when the toner image transferred onto the surface of the intermediate transfer body reaches the secondary transferring position and the timing when the transfer member is conveyed to the secondary transferring position. This may cause a mismatch of the head end of the toner image on the surface of the intermediate transfer body with the head end of the transfer member.
Especially, when an endless belt flexible member made of a material such as synthetic resin or rubber is used as the intermediate transfer body, such a material may be expanded with time due to creep phenomenon. In addition, an error between both timings may become large because expansion and contraction of the material based on the environmental change is large.
In the related art, a technique is suggested where a toner image transferred to an intermediate transfer belt is detected twice by a sensor while the intermediate transfer belt makes a round (one rotation); the variation of the perimeter length of the intermediate transfer belt is determined from the time interval of a detection signal; and a timing for conveying the transfer member to a secondary transfer position is changed according to the variation of the perimeter length of the intermediate transfer belt. See, for example, Japanese Laid-Open Patent Application Publication No. 2001-215857.
However, in the technique discussed in Japanese Laid-Open Patent Application Publication No. 2001-215857, it is necessary to make the intermediate transfer belt go around at least one time in order for the sensor to detect twice the toner image transferred to the intermediate transfer belt. Accordingly, the following problems may occur.
First, since image forming cannot be performed while the intermediate transfer belt makes a round, a time for image forming becomes long so that the productivity may be degraded.
In addition, before the toner image reaches a primary transfer position, it is necessary to remove toner remaining on the surface of the intermediate transfer belt after the previous secondary transfer. However, if the intermediate transfer belt makes a round while the toner image remains on the surface of the intermediate transfer belt, it is necessary to provide a mechanism for switching between removing or not removing the residual toner. This may cause an increase in cost and make the size of the apparatus large.